Since the first installation of passive power filters (PPFs) in the mid 1940's, PPFs have been widely used to suppress current harmonics and compensate reactive power in distribution power systems due to their low cost, simplicity and high efficiency characteristics. Unfortunately, they have many disadvantages such as low dynamic performance, resonance problems, and filtering characteristic that is easily affected by small variations of the system parameters. Since the concept “Active ac Power Filter” was first developed by L. Gyugyi in 1976, the research studies of the active power filters (APFs) for current quality compensation are getting more interest since then. APFs can overcome the disadvantages inherent in PPFs, but their initial and operational costs are relatively high because the dc-link operating voltage should be higher than the system voltage. This results in slowing down their large-scale application in distribution networks. Later on, different hybrid active power filter (HAPF) topologies composed of active and passive components in series and/or parallel have been proposed, aiming to improve the compensation characteristics of PPFs and reduce the voltage and/or current ratings (costs) of the APFs, thus leading to effectiveness in cost and performance.
The HAPF topologies consist of many passive components, such as transformers, capacitors, reactors, and resistors, thus increasing the size and cost of the whole system. A transformerless LC coupling HAPF (LC-HAPF) has been recently proposed and applied for current quality compensation and damping of harmonic propagation in distribution power systems, in which it has only a few passive components and the dc-link operating voltage can be much lower than the APF. Its low dc-link voltage characteristic is due to the system fundamental voltage dropped across the coupling capacitance but not the active part of the LC-HAPF. In addition, the coupling LC circuit is designed based on the fundamental reactive power consumption and the dominant harmonic current of the loading. And the active part is solely responsible for the current harmonics compensation. Therefore, this LC-HAPF can only inject a fixed amount of reactive power which is provided by the coupling LC and thus achieving a low dc-link voltage level requirement in this special situation. In practical, because the load-side reactive power consumption varies from time to time, the LC-HAPF cannot perform satisfactory dynamic reactive power compensation. The larger the reactive power compensation difference between the load-side and coupling LC, the larger the system current and loss, and it will lower the power network stability. In addition, if the loading is dominated by a centralized air-conditioning system, its reactive power consumption will be much higher than the harmonic power consumption. Therefore, it is important and necessary for the LC-HAPF to possess dynamic reactive power compensation capability under this loading situation.
Besides, the LC-HAPF and other HAPF topologies are all operating at a fixed dc-link voltage level. Since the switching loss is directly proportional to the dc-link voltage, the system will obtain a larger switching loss if a higher dc-link voltage is used, and vice versa. Therefore, if the dc-link voltage can be adaptively changed according to different loading reactive power situations, the system can obtain better performances and operational flexibility.
There is a need for the LC-HAPF providing dynamic reactive power compensation capability with reducing switching loss and switching noise purposes.